Per IEEE C57.149, testing with oil is the most common and preferred method for frequency response analysis. Special consideration should be given to safety when testing a transformer without oil so that excessive voltages are not applied. Presence of oil changes the frequency response. Results with and without oil will cause variations in the SFRA traces. Below is an excerpt from the IEEE guidelines:
"For new equipment, this my require the performance of two FRA tests after receipt of the equipment at the final destination: 1) one test with the transformer in its shipping configuration, 2) and one thest with the transformer assembled an oil-filled as required for insulation resistance testing, to be used as baseline data for future testing. If no shipping damage is suspected, the test in the as shpped configuration may not be necessary as a receipt test"
Often, the manufacturer fills and drains the transformer before shipment. You should be aware of the conditions in which an SFRA test was performed before shipment from the factory. IEEE further states that:
“If the equipment is to arrive drained of oil, the shipping configuration should specify that it will be tested pre and post movement without oil. If the equipment is to be shipped after being drained of oil, it should be tested pre-movement without oil. Testing the unit prior to shipment in this case without oil and prior to a first fill, may not be adequate and could lead to false failures due to residual oil being held in the windings, or additional oil draining from the winding during weeks of shipment. If the equipment is to be shipped with oil, it should be fully filled for both pre and post movement tests. If the equipment is to be shipped partially filled, it should be tested with the same level of oil, or preferentially after oil has been added. Ensuring oil is at the same level before and after transportation for partially filled transformers can be difficult and sometimes leads to incorrect assessments.”